Vidic has been at Old Trafford for seven years, establishing himself as one of the Premier League's best defenders, but a series of injuries have prompted concerns within the club about whether he will be able to return to his imperious best.

He has featured in only 18 of a possible 44 club games so far this season, the first leg of his side's crucial Champions League clash with Real Madrid among his higher-profile absences.

Added to his contractual situation and age of 31, United manager Sir Alex Ferguson would be willing to entertain offers of £10 million or above for his skipper, despite any sale likely to prove unpopular with fans.

Milan are seeking a new man to partner Philippe Mexes at the centre of their back defence, with Bartosz Salamon, 21, lacking the experience to be considered ready to start on a regular basis.

Massimiliano Allegri's side have their eyes on alternative targets should Vidic prove unattainable, mostly based in Serie A, with Davide Astori of Cagliari and Torino's Angelo Ogbonna the most prominent.

The likes of Mehdi Benatia and Danilo, both of Udinese, are seen as second-tier options by club observers, while youngster Kurt Zouma of St Etienne is also on the club's radar.